Field of the Invention
The present invention relates to a method for controlling the torque of a pump of a type frequently used for construction machines and other similar machines in which regulators are driven by a plurality of pumps.
FIG. 6 is a schematic drawing showing a configuration of a conventional engine pump using two pumps. Variable delivery pumps 2a and 2b include swash plates 3a and 3b, which vary the discharge flow rates of the pumps are driven by an engine 1. Swash plates 3a and 3b are provided with are driven by regulators 4a and 4b. An operation lever 5 provides pilot pressure that varies in proportion to its operating position through lines 6a and 6b to regulators 4a and 4b. One or both swash plates 3a and 3b may be driven in dependence on the setting of operation lever 5. Since the apparatus of FIG. 6 is conventional, further explanation of the selecting circuit for this operation is omitted.
Swash plates 3a and 3b are controlled by pilot pressure from lines 6a and 6b in response to the position of operation lever 5 as well as a discharge pressure of pumps 2a and 2b themselves. The change of position of the swash plates is limited by restricting power so that the change of position does not exceed the output power of the engine when load is applied, i.e., when discharge pressure is great.
Referring now to FIG. 3, conventionally, the total output power of a pump is set, allowing a margin of the design engine output. This permits the engine to operate within the governor area. The governor area means the range of engine output torque variation in output torque is capable of controlling the engine speed over a relatively narrow range. Point B is located at the upper limit of the governor area. Below point B, the engine speed can be controlled by varying rack displacement, as long as rack displacement remains less than the maximum rack displacement. At the maximum rack displacement, the speed is in the lagging area.
Fuel consumption characteristics (in the figure, higher engine speed indicates less economical fuel consumption) is not favorable with the engine speed at point B, suggesting that it is not operating efficiently.
Further, the output of each pump is a fixed value. Therefore, when one of the two pumps is driven, for example, less than half the engine power is used.
In short, the prior art presents the following drawbacks:
(1) the engine is operated under unfavorable, inefficient conditions with respect to fuel consumption, and
(2) available engine power is not used to its full extent.
Another example of a controlling device is disclosed in Japanese Patent Application Laid-Open No. 50686/1988 that calls for establishing a pump absorption characteristic which makes a pump do a specified amount of work based on engine speed. This is accomplished by controlling the pump swash plates (in other words the pump discharge flow rate) according to the pump power absorption characteristic and the pump discharge pressure.
As shown in FIG. 3, engine output characteristics vary considerably, depending on whether the engine speed is in the governor area, where the engine speed can be maintained at more or less a fixed level regardless of a small change in engine output torque, or in the lagging area, where such control is ineffective. Therefore, while it is necessary to switch the pump absorption torque in accordance with the area in which the rack displacement is located, the control device presented in the above Japanese Patent Application Laid-Open No. 50686/1988 does not solve this problem.